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<a name="C_002fC_002b_002b-Language-Constructs-for-TM"></a>
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<a name="C_002fC_002b_002b-Language-Constructs-for-TM-1"></a>
<h2 class="chapter">2 C/C++ Language Constructs for TM</h2>

<p>Transactions are supported in C++ and C in the form of transaction statements,
transaction expressions, and function transactions. In the following example,
both <code>a</code> and <code>b</code> will be read and the difference will be written to
<code>c</code>, all atomically and isolated from other transactions:
</p>
<div class="example">
<pre class="example">__transaction_atomic { c = a - b; }
</pre></div>

<p>Therefore, another thread can use the following code to concurrently update
<code>b</code> without ever causing <code>c</code> to hold a negative value (and without
having to use other synchronization constructs such as locks or C++11
atomics):
</p>
<div class="example">
<pre class="example">__transaction_atomic { if (a &gt; b) b++; }
</pre></div>

<p>GCC follows the <a href="https://sites.google.com/site/tmforcplusplus/">Draft
Specification of Transactional Language Constructs for C++ (v1.1)</a> in its
implementation of transactions.
</p>
<p>The precise semantics of transactions are defined in terms of the C++11/C11
memory model (see the specification). Roughly, transactions provide
synchronization guarantees that are similar to what would be guaranteed when
using a single global lock as a guard for all transactions. Note that like
other synchronization constructs in C/C++, transactions rely on a
data-race-free program (e.g., a nontransactional write that is concurrent
with a transactional read to the same memory location is a data race).
</p>




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